Cordierite has been used for a variety of applications such as catalytic substrates and filters for diesel particulate emission. Cordierite has low thermal expansion and is therefore suited for applications where high thermal shock resistance is required. Cordierite shows anisotropy in its thermal expansion with the different crystallographic directions exhibiting positive and negative expansion. Due to the anisotropy in thermal expansion, mismatch strains build up between grains with different crystallographic orientation, and such strains can lead to microcracking. Polycrystalline cordierite ceramics undergo extensive microcracking during thermal cycling. Microcracks open during cooling and heal during heating. This creates a hysteresis response in the thermal cycling behavior with differences between the heating and cooling curve that can be attributed to the presence of microcracks. As a result of the microcracking, the overall thermal expansion of the ceramics is lowered compared to the crystallographic average CTE.
In one sense, lowering the coefficient of thermal expansion (CTE) through microcracking is beneficial, as the thermal shock resistance of the material, which is proportional to the material's strength and inversely proportional to its elastic modulus and thermal expansion, is expected to improve with microcracking. However, the material strength is significantly lowered with growing microcrack density, so that balancing fracture toughness, porosity, thermal expansion and strength becomes difficult.